This invention relates to transformers and, more particularly, to transformers which may be employed in gas discharge lamp ballast circuits.
Gas discharge lamps are unusual electrical devices primarily because they exhibit a negative resistance current-voltage characteristic. That is to say, once the arc within the lamp has been initiated, an increasing level of current between the lamp electrodes actually results in a decrease in the voltage across these electrodes. The ballast circuit which supplies electrical energy to such lamps, therefore, must usually provide some means of limiting the current through the lamp or lamps. As seen in the text, Electric Discharge Lamps by Waymouth (pages 317-318), current may be limited by providing a separate inductor or by employing an E-core transformer with primary and secondary legs and a third gapped leg acting as a magnetic shunt path. However, such E-cores are open structures and exhibit high levels of stray magnetic flux, and for reasons which are discussed below, such "leaky" transformers are inappropriate for the purposes described herein.
While it is possible to employ a separate, current-limiting inductor, such circuit construction designs are typically large, heavy and uneconomical in that more core transformer core material is employed than is necessary. Nonetheless, current limitation is a highly desirable, if not essential, feature of gas discharge lamp ballast circuits. This is because such lamps exhibit a negative resistance characteristic after the lamp arc has been struck (that is initiated) which permits the current to increase to destructive levels if not checked by ballast circuitry.
The above-mentioned text by Waymouth reinforces these lamp ballast design features and further indicates that leakage reactance may be added in an autotransformer configuration to reduce weight and cost. In particular, he indicates that a transformer may be conventionally constructed with primary and secondary windings wound on outer legs of conventional transformer cores with an inner leg providing a desired flux leakage path. However, there is no provision in these conventional designs to prevent the occurrence of stray magnetic flux leakage.
The problem of stray magnetic flux leakage is an important one. Gas discharge lamps, being driven by alternating current, accordingly produce time varying magnetic fields. These changing fields induce electric currents in surrounding conductive structures. Such structures are conventionally found in gas discharge lamp fixtures, luminaires and nearby electronic ballast circuitry. These induced currents cause heating and a loss in overall system efficiency. To eliminate these stray magnetic fields, flux shields, exhibiting high electrical conductance, must generally be employed. These flux shields add not only to the weight but also to the cost of the resulting ballast circuit.
Several U.S. patents address the problem of magnetic flux leakage and integrated transformer-inductor designs. In particular, U.S. Pat. No. 3,703,677 issued to Victor Farrow describes a conventional E-core with a gapped center leg where the leakage inductance is obtained by physical separation of the primary and secondary windings. He also describes the same problems discussed above with respect to magnetic flux leakage which tends to induce eddy current losses in surrounding metal structures. Because leakage flux may be reduced by placing the primary and secondary windings physically closer together, an alternate embodiment of the gapped center leg E-core design has also been suggested. In this transformer design, an outer transformer leg is gapped. Nonetheless, the problem of stray magnetic flux leakage occurs because of the inherently unshielded nature of the E-core transformer design.
Even though the magnetic shunt may considerably reduce the stray flux, there is still sufficient stray flux in these E-core designs to cause three important problems. First, the residual stray flux still produces eddy current losses which means that the transformer must be contained within a high conductivity flux shield. Second, stray flux creates a leakage inductance which is in addition to the leakage inductance created by the controlled leakage flux through the magnetic shunt. Thirdly, when a dual primary winding is used with circuits employing two power semiconductors in the output stage, such as is done in so-called push-pull circuits, the stray flux creates undesirable leakage inductance between these two primary windings. It is these last two problems which cause integrated transformers and inductors designed using E-cores to be larger than desirable. More specifically, the leakage inductance varies as the square of the number of turns. To keep the leakage inductance due to stray flux within acceptable limits, the designer is forced to use a relatively small number of turns on the primary and secondary. However, since the input voltage and lamp voltage are fixed, this creates a large voltage per turn on primary and secondary windings for transformers used in electronic gas discharge lamp ballast circuits. The magnetic field intensity in the transformer core is proportional to this voltage per turn divided by the core cross-sectional area. Since these high magnetic field intensities produce high loss in the material and may even saturate the material, the high voltage per turn caused by using a small number of turns forces the designer to use cores of larger cross-sectional area to prevent high magnetic field intensities from being created. Thus, the requirement for large cross-sectional area in turn increases the size and weight of the entire integrated transformer-inductor.
Some problems with stray magnetic flux have been alleviated in the past through the use of so-called pot-cores as transformer forms. However, until now, such core structures have never been employed in an integrated construction in which magnetic flux shunt paths are employed to produce an integrated transformer-inductor structure.
In certain inductors, particularly those operating at the normal 60 hertz line frequency, there has been observed a tendency on the part of some core structures to vibrate, thus causing accoustic noise and loss of efficiency. One inductor structure which has been suggested to solve this accoustic problem employs a pot-core-like structure with a center post disposed at least partially through an opening in a washer-shaped core end closure member. The purpose of this structure is to provide symmetry so that magnetic forces act uniformly on the center post in all directions so as to prevent osillatory forces from causing vibrations. However, such devices do not employ transformer structures and are not designed to incorporate magnetic leakage flux inductance.